1. Field of the Invention
The present invention is generally related to imaging and non-imaging optical preforms utilized, for example, in the fabrication of optical fibers, emitters, and sensors and, in particular, to the formation of a unique optical preform having a deeply placed radially bonded interface layer of controlled radial depth and symmetry.
2. Description of the Related Art
There are numerous applications and methods of producing optical preforms. Some of the more common preform applications include use as the source component for the drawing of optical fiber, as bulk source material for lens blanks, and as the cap or encapsulating lens of optical emitters. In these and other uses, the optical, mechanical and thermal properties of the optical preform and the precise definition of these properties is greatly valued. Furthermore, a graded variation of these properties within the finished product, either through structural or material processing, is also greatly desired.
At least three primary methods of fabricating optical preforms relevant to the present invention are conventionally known. The first is the use of chemical vapor deposition (CVD) to deposit a material on the interior surface of a glass tube. The object of this process is to provide a core portion of a material having a first set of optical characteristics surrounded by a cladding layer having a second set of optical characteristics. Creation of the optical preform requires the use of a cladding tube formed from a high purity silica based glass, typically composed of greater than 95% silica, with a small amount of a low diffusivity dopant added to establish the optical characteristics of the cladding. The low diffusivity of the dopant is required to minimize thermal migration if not direct loss of the dopant during processing. A high purity silica vapor, though also containing a low concentration of a selected dopant, is then pumped through the cladding tube while the cladding is heated in a zone that is mechanically moved repeatedly along the length of the cladding to facilitate the actual deposition of silica and dopant from the vapor phase onto the interior surface of the cladding. Selection of the vapor phase dopant material and its concentration, and thereby the optical characteristics of the core material formed by deposition, is particularly limited by the requirement of uniform deposition of the dopant relative to the deposited silica. In addition, temperatures and flow rates must be further carefully maintained to achieve the uniform deposition of the silica while retaining a uniform dopant concentration in the original and deposited material. Precise temperature control is also required so as not to overheat the cladding, resulting in asymmetrical deformation that would, in turn, compromise the desired geometric structure of the optical preform. Once a layer of the core silica material has been deposited, a high temperature treatment must be uniformly applied to the cladding and core to collapse the entire structure as necessary to fill the center of the preform.
The CVD process is not only costly and complex due to the required precision at many process steps, but the process is quite time intensive since the rate of uniform vapor phase deposition is inherently low. Perhaps the most significant limitation, however, is that substantial materials limitations are present due to the fundamental nature of the process. In particular, the cladding and vapor deposited core materials are required to be of the same elemental glass composition, conventionally referred to as being of the same glass "family." Examples of conventional glass families include borosilicate glasses, lead glasses, and barium glasses. By virtue of the core and cladding being of the same glass family and the ratio of dopants to silica in both being quite low, the difference in material properties between the core and cladding is inherently limited. For example, CVD preforms are substantially limited to a core to cladding difference in index of refraction of about 0.1 and more typically 0.03 or less.
Also, the thermal and mechanical properties of the vapor deposited core and cladding materials are highly interdependent in order to perform correctly in the final collapse stage of the process so that undue strain is not placed on the cladding material. Consequently, the optical, mechanical and thermal properties of the preform fabricated in a CVD process are significantly limited.
Another process for forming optical preforms uses ion diffusion to alter the surface optical, mechanical and thermal properties of an otherwise homogeneous optical material rod. In this process, the rod is placed in an ion salt bath and heated to a temperature sufficient to encourage ion transport at the surface of the rod material. In effect, a leaching of the surface material occurs resulting in an alteration of the material properties within the leached zone. This zone can be formed to a substantial radial depth, though only at the surface of the rod material. As a practical matter, however, the zone can achieve a radial depth of only a fraction of a millimeter to several millimeters during a leaching period of about three to four months. Furthermore, due to ion transport mechanics being highly dependent on the specific ion concentration at the surface of the optical rod material, precise control of the resulting optical characteristics is quite difficult. The leaching action also directly reduces the material strength and integrity of the preform in the affected zone. Consequently, the finished optical properties of the resulting preform may vary to a degree that is not commercially acceptable for many optical preform uses.
Finally, a third method for forming an optical preform is to simply collapse a cladding tube of an optical material onto a rod of the same or different optical material. U.S. Pat. No. 4,486,214, issued to Lynch on Dec. 4, 1984, discloses an example of this process. The object of the preform fabrication process disclosed there is to create a preform having a sharply defined change in index of refraction between the inner rod and outer tube material. Although not as limited as in CVD processes, the choice of materials for the outer tube is limited by the requirement that the outer tube collapse uniformly onto the rod without deformation of the rod material. The thermal and mechanical characteristics of the collapsing tube must therefore be chosen to allow a uniform collapse at a temperature appropriately below the melting temperature of the rod material. The collapsed tube is ultimately fused to the rod material not as part of the formation of the preform, but only during a separate subsequent step of drawing the optical preform into optical fiber. The simultaneous step of fusing and drawing the fiber allows the abrupt optical interface between the tube and rod material to be sharply maintained, the precise goal of preforms made by this process. However, materials fusion at the drawing stage also maintains any impurities and gaps that may exist at the material interface. Consequently, a premium is placed upon the initial precise collapse of the tube on the rod in performance of this process.